Directional couplers are used for sampling a signal in one direction only. Ideally, the signal is sampled without changing the signal's characteristics, in one propagating direction only. FIG. 1 schematically depicts a directional coupler. Four signal ports are shown: “input,” “output,” “coupled” (a.k.a. “sampled”) and “isolated.” The isolated port is sometimes placed inside the physical device and is often terminated in such a way as to prevent a reflected signal from reaching the other three ports. For example, the isolated port may be terminated with a matched load. Although it is possible to create a directional coupler using lumped elements, such as capacitors and mutual inductors, most practical circuits are realized by using coupled transmission lines.
Directional couplers may be understood by considering the modes of transverse electromagnetic (TEM) propagation. Whenever it becomes possible for oppositely polarized current pairs to flow in a structure, that structure may be said to support a TEM mode. A pair of conductors can support one TEM mode while three conductors can support two independent TEM modes. It can be shown that N+1 conductors can support N independent TEM modes of propagation, and that all TEM propagation can be modeled as a linear combination of the N independent modes. Thus, three conductors can be arranged so that driving one mode of propagation results in a second electromagnetically induced mode of propagation. This induced mode is the so-called “coupled” or “sampled” mode.
One measure of a coupler's ideality is the coupler's directivity. Directivity is a ratio of (a) the forward sampled power (i.e. the power traveling from the input port to the output port) to (b) the reverse sampled power (i.e. the power traveling from the output port to the isolated port), when the power is intended to be sampled from the forward traveling signal and the forward and reverse traveling signal amplitudes are equal. Among the factors that can spoil a coupler's directivity is a difference in phase velocity between modes. In the case of microstrip directional couplers, which are normally comprised of coupled transmission lines mounted on a printed wiring board (“PWB”), this problem becomes more pronounced as the relative permittivity (∈r) of the PWB is increased.
FIGS. 2a and 2b illustrate the problem created by the relative permittivity of the PWB. In FIG. 2a, there is shown a three-conductor coupled microstrip in the so called “even mode”, in which two of the conductors 10, 20 are either charged to a negative potential or a positive potential and the third conductor 30 is neutral. In FIG. 2a, two of the conductors 10, 20 are charged to a positive potential. In FIG. 2b, there is shown the same coupled microstrip in the so called “odd mode”, in which one of the conductors 10 is positively charged, another of the conductors 20 is negatively charged, and the third conductor 30 is neutral. Dotted areas symbolize the dielectric material of the PWB. In the even mode, the E-field is concentrated in the PWB, which has a higher ∈r than air, and the phase velocity of the even-mode signal is lower than the free-space velocity due to this dielectric loading. In the odd mode, some of the E-field maps into the air region above the PWB, raising the phase velocity of the odd-mode signal compared to the even mode. So, because the E-field in the even mode disperses differently than the E-field in the odd mode, there is a difference in phase velocity between the even and odd modes. As such, the load at the isolated port may be matched for one of the modes (even or odd) but not the other mode. An unmatched load for one or both modes results in energy being reflected by the port corresponding to the unmatched load, resulting in non-ideal behavior, e.g., poor isolation. Ideally, each port of the directional coupler would have a matched load for all propagating modes.
Techniques for compensating for the odd-mode phase velocity of a microstrip directional coupler are well known. Such techniques include placing capacitors at the ends of the microstrip directional coupler. Since it takes time to charge and discharge capacitors, the odd-mode phase velocity is effectively retarded by incorporating capacitors at the ends of the coupled microstrip section.
A directional coupler is a reactive device, and thus has a finite operational bandwidth. The operational bandwidth is the frequency range over which a coupler is considered to accurately indicate characteristics of a signal at a particular power level. In order to increase the operational bandwidth, multiple sections of coupled lines are often added in series in order to expand the operational bandwidth. However, when an existing multi-sectioned directional coupler is operated in the odd mode, the phase velocity difference associated with one of the sections is different from the phase velocity difference associated with another of the sections. The dissimilar phase velocities associated with each of the sections makes it difficult to compensate for the phase velocity difference between even and odd modes across the desired operational bandwidth of the multi-sectioned coupler. Consequently, the difference in phase velocity between modes makes it difficult for microstrip couplers to compete with other types of coupled transmission lines (e.g., strip line).
Microstrip fabrication techniques are simpler and cheaper than other fabrication techniques (e.g. manufacturing techniques used to manufacture stripline couplers). So, there is a strong incentive to try to ameliorate the problems caused by the phase velocity difference between the even and odd modes in a multi-sectioned microstrip directional coupler.